1. Field of the Invention
The present invention relates to improvement of an electrolyte solution for an electrolytic capacitor and an electrolytic capacitor using the same, particularly to an electrolyte solution for an electrolytic capacitor to which specific inorganic fine particles having a specific surface structure are added, which can maintain a high electric conductivity and a high voltage-resistance for a long time and an electrolytic capacitor using the same.
2. Prior Art
An electrolytic capacitor is generally provided with an insulating oxide film such as aluminum oxide and tantalum oxide which is formed as a dielectric layer on an anode, and also provided with a cathode so that it faces the anode, both electrodes being interposed by a separator which retains an electrolyte solution.
Said anode is usually subjected to an etching treatment to enhance the surface area. The electrolyte solution closely contacting with the concave and convex surface of the electrode functions as a substantial cathode. Thus, electric conductivity and temperature characteristics of the electrolyte solution become main factors to determine the electric characteristics of an electrolytic capacitor.
A voltage at which the insulating oxide thin film is destroyed with the increase of the load voltage on the capacitor is generally regarded as a measure of the voltage-resistance of the capacitor and so called spark voltage. If the spark voltage is higher, the voltage-resistance of the capacitor is higher. The spark voltage depends on the composition of the electrolyte solution used. Therefore, the electrolyte solution is an important element which determines the properties of the electrolytic capacitor.
Conventionally, for the purpose of increasing the spark voltage while less decrease in the electric conductivity of the electrolyte solution, adding oxide fine particles to an electrolyte solution for an electrolytic capacitor has been known. For example, an electrolyte solution in which silica fine particles are added (Japanese Provisional Patent Publication No. 12512/1992) and an electrolyte solution in which aluminum oxide, zirconium oxide, antimony oxide, tantalum oxide or titanium oxide is added (Japanese Provisional Patent Publications Nos. 145612/1992 to 145616/1992) are described.
However, an electrolyte solution comprising such fine particles has a disadvantage that it cannot maintain the high voltage-resistance at a temperature as high as 105.degree. C., since the colloid is unstable at such high temperatures and resulting the short life of a capacitor. It has another disadvantage that a small amount of water present in the electrolyte solution, for example, from moisture or produced by the esterification reaction of acid solutes and alcohol solvents accelerates the gelation of the electrolyte solution, resulting in the decrease of the voltage-resistance.